Aperture

Light enters a camera through a small hole in the front of the camera.  The size of this hole can be controlled and is referred to as the ‘aperture’. The bigger the aperture, the bigger the hole, and the faster light can enter the lens. If we think of shutter speed as controlling the time for which light enters the camera, then we can think of aperture as controlling the rate at which light enters the camera.

Aperture is unfortunately a more complex subject than shutter speed. It is also generally more important than shutter speed, and so I recommend you spend a bit of time playing around with the aperture settings on your camera, to get a feel of how it works and what changes when you increase or decrease the aperture.

The best way to change your camera’s aperture is to use “A” (Aperture) mode.  In this mode you can adjust the aperture (usually by rotating a wheel on your camera) while the camera will automatically attempt to adjust the shutter speed to keep a good exposure.  You can of course use “M” mode, which will give you complete control over the shutter speed and aperture.

If you remember when we discussed focus, we introduced a concept called “depth of field”. The depth of field refers to the distance over which objects will appear in focus. A shallow depth of field means that only objects close to the focal distance will appear in focus, whereas a deep depth of field will mean even objects far from the focal distance are in focus. Depth of field is an important concept in photography, and will be covered in more detail in a later lesson.

Controlling aperture is important in capturing portraits of people or animals. In this shot the monkey is sharp and in focus while both the cable in the foreground and the forest in the background are blurred. This is done by using a low aperture, and so a low depth of field.

Depth of field is controlled by aperture. A small aperture will result in a deep depth of field, whereas a large aperture will result in a shallow depth of field. The photo above shows a shallow depth of field – only the parts of the image close to the focal distance (i.e the monkey) are in focus while everything else is blurred. On the contrary, in a photograph with a deep depth of field almost everything in the image should appear in focus.

As we saw with shutter speed, aperture is measured in stops. Increasing one stop doubles the amount of light entering the camera, and decreasing one stop halves the light. With shutter speed this was quite easy to calculate. Unfortunately aperture is not quite so simple. The reason is that we need to consider the area of the hole, and this increases with the square of the radius. Doubling the radius of the hole will increase the area four times, or an increase of two stops. An increase of one stop actually means the radius has increased by 1.4 times (the square root of 2).

Aperture is measured in terms of the radius of the hole. This is done using “f-stops” and these are usually labelled as f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and so on. Depending on your camera and lens you may find some aperture values between these values (these are typically half and third stops). Regardless, the series above shows the stops needed to increase or decrease the exposure by two times. Going from f/2 to f/2.8 will half the exposure, while going from f/8 to f/5.6 will double the exposure (the higher the number the smaller the hole!). These numbers are tricky to try and calculate, so it’s good to try and memorise them.  For now just remember that a low value like f/1 or f/1.4 means a very wide aperture (and therefore a shallow depth of field), while a high number like f/16 or f/22 means a very narrow aperture (and a deep depth of field).

Decreasing the aperture too much can have some negative effects on your photos. Depending on the construction of your lens a narrow aperture can start to produce blurring in your photographs. Although it differs for all lenses, in most lenses the optimal aperture for photo quality is around f/8. This should be where you keep the aperture if you don’t have any reason to have a shallow or deep depth of field.

One area where a wide aperture (f/4 or f/2.8 or even below) is useful is portrait photography. By using this technique you will produce a shallow depth of field, which will keep the face sharp and in focus but leave the background blurred. You can purchase special lenses for portrait photography that allow you to use much wider apertures than are possible with standard lenses. The image below shows how aperture can be used to emphasise the foreground.

An example of how aperture can be used to isolate the foreground of an image.

The topic of aperture can be quite confusing, especially as the terminology is quite difficult. I recommend you spend some time experimenting with the aperture settings on your camera, and spend a bit of time going over the “f-stop” notation again.

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